System and method for cache entry aging

ABSTRACT

A system comprises a host device and a cache controller. The host device includes a command buffer and a host application that posts a cache command that includes a cache key and a key aging alias in the command buffer. The cache controller includes logic circuitry configured to load the cache command from the command buffer of the first host device into the buffer memory, identify a match, if any, for the cache key in the command queue, perform the cache command, and return cache completion status information to the first host application, wherein the cache completion status information includes a value of the key aging alias in cache metadata when a match for the cache key is found and includes a value of the key aging alias provided by the first host application when a match for the cache key is not found.

CLAIM OF PRIORITY

This Application is a Continuation Application of U.S. application Ser. No. 14/458,606, filed Aug. 13, 2014, the contents of which are hereby incorporated by reference in its entirety and the benefit of priority is claimed herein.

TECHNICAL FIELD

The present disclosure relates generally to networked storage systems, and more particularly, in some embodiments, to subsystems for facilitating data storage and access in flash based networked storage systems.

BACKGROUND

The large amounts of information generated daily challenge data handling facilities as never before. In the context of today's information generation, data is being generated at rates perhaps thousands or tens of thousands of times greater than was the data-generation rate in the 1990s. Historically, large volumes of data sparked explosive growth in data communications. Responses to growing amounts of data generation centered on improving the movement of data based in increased transmission data rates to enhance throughput in communication channels. For instance, transmission pipelines grew from a few tens of megabits-per-second (Mb/s) transmission rates to several tens of gigabits-per-second (Gb/s) rates during the 1990s.

In the same period, typical storage devices, such as hard disk drives (HDDs), when amassed in sufficient numbers, might accommodate large volumes of data, but the rates at which data may be stored and retrieved have not scaled at the same rate as the volume of data stored on the devices has increased. Data access rates for HDDs are at similar orders of magnitude today as they were in the 1990s.

Fundamental storage subsystems have not integrated technology to enable scaling of effective data storage at the same rate that data generation is growing. Hence the challenge to systems handling large volumes of data is not likely to be alleviated by the combination of contemporary HDD technology with high-speed data transmission channels. In order to handle and manage big data, information processing facilities will be pressured to utilize larger volumes of storage with higher performance rates for capturing and accessing data.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings in which:

FIG. 1 is an illustrative schematic block diagram of a network storage system according to some embodiments;

FIG. 2 is a block diagram showing a plurality of Flash storage modules coupled to a Flash memory controller, which in turn, is coupled to the first and second packet routing networks according to some embodiments;

FIG. 3 is a block diagram showing a dual inline memory module containing DRAM and flash memory, as may be used in example embodiments;

FIG. 4 is a block diagram showing an example of interfaces to a cache controller according to some embodiments;

FIG. 5 shows a flow diagram of an example of a method of managing access to cache memory using two cache controllers according to some embodiments;

FIG. 6 is a block diagram of portions of an example of a cache controller according to some embodiments;

FIG. 7 shows a diagram of an example of a sequence of communications for loading cache commands in cache controllers according to some embodiments;

FIGS. 8 and 9 show diagrams of examples of a sequence of communications for cache read commands performed by a cache controller according to some embodiments;

FIGS. 10 and 11 show diagrams of examples of a sequence of communications for cache write commands performed by a cache controller according to some embodiments;

FIG. 12 shows an example of a format for cache commands according to some embodiments;

FIG. 13 shows an example of a format for a completion status word according to some embodiments;

FIG. 14 is a flow diagram of an example of a method of implementing cache entry aging according to some embodiments;

FIG. 15 illustrates the interaction of components of cache management during a cache miss according to some embodiments;

FIG. 16 illustrates the interaction of components of cache management during a cache hit according to some embodiments;

FIGS. 17A, 17B, and 17C illustrate example flow diagrams showing caching principles, policies, or rules implemented by the cache management of the system according to some embodiments.

DESCRIPTION OF EMBODIMENTS Introduction

The following description is presented to enable any person skilled in the art to create and use a computer system that provides high speed access to data storage devices, particularly Flash storage devices. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, in the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention might be practiced without the use of these specific details. In other instances, well-known data structures and processes are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Identical reference numerals may be used to represent different views of the same item in different drawings. Flow diagrams in drawings referenced below are used to represent processes. A computer system is configured to perform some of these processes. The flow diagrams that represent computer-implemented processes include modules that represent the configuration of a computer system according to computer program code to perform the acts described with reference to these modules. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

System Overview

FIG. 1 is an illustrative schematic block diagram of a network storage system 100 in accordance with some embodiments. The system includes redundant components that permit recovery from component failures, for example. More specifically, the system 100 includes redundant first and second packet routing networks, referred to as the “routing networks” or “networks,” 102-1, 102-2 that route packets between endpoints. First and second general purpose management processors 116-1, 116-2 are coupled as endpoints to each of the networks 102-1, 102-2. The first and second general purpose management processors 116-1, 116-2 may be implemented as one or more server boards and are associated with respective first and second non-transitory local memory devices (hereinafter “local memory devices” or “local memory”) 117-1, 117-2. A plurality of Flash solid state (storage) drive (SSD) circuits (hereinafter “Flash circuits” or “Flash”) 110-1 to 110-8 are coupled as endpoints to each of the routing networks 102-1, 102-2. Input/Output (I/O) circuits 112-1 to 112-4 are coupled as endpoints to each of the routing networks 102-1, 102-2 and act as communication interfaces between the routing networks 102-1, 102-2 and an external network 104. Storage access networks 108-1 through 108-4 provide access to the Flash circuits 110-1 through 110-8. Storage I/O interface circuits 114-1 to 114-4 are coupled as endpoints to each of the routing networks 102-1, 102-2 and act as communication interfaces between the routing networks 102-1, 102-2 and storage access networks 108-1 through 108-4. First and second packet processing circuits 118-1, 118-2 are coupled as endpoints to each of the network 102-1, 102-2. The first and second packet processing circuits are configured to impart one or more services to packets transported over the networks 102-1, 102-2. In some embodiments, the service imparted by the first and second packet processing circuits includes a cache service, and the first and second packet processing circuits 118-1, 118-2 are coupled to cache storage circuitry 106-1, 106-2, which in some embodiments includes Flash-backed DRAM circuits. In some embodiments, the service imparted by the first and second packet processing circuits includes one or more of encryption/decryption, duplication/de-duplication, compression/de-compression, replication, and snapshot, for example.

In accordance with some embodiments, the network “fabric” of the first and second packet routing networks 102-1, 102-2 is compliant with the PCI Express Base Specification (hereinafter “PCIe”) released by the PCISIG (PCI Special Interest Group). See, PCI Express Technology, Comprehensive Guide to Generations 1.x, 2.x and 3.0, by M. Jackson and R. Budruk, 2102, Mindshare, Inc. PCIe specifies point-to-point bidirectional serial communication paths between endpoints over switches and connection lines. Information is transmitted in packets between endpoints over the routing networks 102-1, 102-2. A PCIe network includes serial connection lines 103-1, 103-2 commonly referred to as “links” that are capable of sending and receiving information at the same time. More specifically, information transmitted through either one or the other of the routing networks 102-1, 102-2 is encapsulated in packets that include routing information that indicates a source endpoint and a destination endpoint. According to the PCIe specification and in accordance with some embodiments, a link can include one or more serial transmit and serial receive connection pairs. Each individual pair is referred to as a “lane.” A link can be made up of multiple lanes. Each lane uses differential signaling, sending both positive and negative versions of the same signal. Advantages of differential signaling include improved noise immunity and reduced signal voltage. Each endpoint device coupled to one or both of the routing networks 102-1, 102-2 includes “core” logic that implements one or more functions. A device that is a component of a typical PCIe compliant network can have multiple functions, up to eight in some embodiments, each implementing its own configuration space.

The first management processor 116-1 is used to configure the first packet routing network circuit 102-1 to provide point-to-point communication between components operably coupled to it. The second management processor 116-2 is used to configure the second packet routing network circuit 102-2 to provide point-to-point communication between components operably coupled to it. In some embodiments, the first and second management processors 116-1, 116-2 configure point-to-point routing within the first and second packet routing networks. In other words, for a given pair of resource circuits, a fixed route among switches in the internal network circuits 102-1 or 102-2 is configured to transmit packets between the pair.

The PCIe specification specifies use of a root complex to configure a PCIe compliant network. A root complex includes interface circuitry (e.g., processor interface, DRAM interface) that couples a management processor and the rest of a PCIe network. Management processor 116-1 includes first and second root complexes 122-1, 122-2 that act as interfaces between processor 116-1 and network circuits 102-1 and 102-2. Management processor 116-2 includes second and third root complexes 122-3, 122-4 that act as interfaces between processor 116-2 and network circuits 102-1 and 102-2. The term “root” is used to indicate that the root complex is disposed at a root of an inverted tree topology that is characteristic of a hierarchical PCIe compliant network.

FIG. 2 is an illustrative schematic diagram showing a plurality of Flash storage modules 113-1 to 113-N coupled to a Flash memory controller 180, which in turn, is coupled to the first and second packet networks 102-1, 102-2. It will be appreciated that for simplification of explanation, FIG. 1 shows the system 100 with only a few representative Flash circuits 110-1 to 110-8. However, in some embodiments, a system 100 can include a large number “M” of Flash endpoints, such as two-hundred Flash endpoints for example. Moreover, as shown in FIG. 2, in some embodiments each endpoints 110-1 to 110-N can include a double stacked Flash storage module that includes multiple individual Flash integrated circuits (ICs) 182 arranged on opposite surfaces of a printed circuit board (PCB) (not shown). In accordance with some embodiments, the Flash circuit modules 110-1 to 110-N share the same form factor, and in some embodiments, the Flash circuit modules' form factor is compliant with a dual inline memory module (DIMM) format. A memory controller 180 is coupled to the routing networks 102-1, 102-2 and manages the flow of data going to and from the Flash ICs on the Flash storage module of the endpoints.

Referring again to FIG. 1, the I/O interface circuits 112-1 to 112-4 provide high-speed connections between the external network 104 (e.g., InfiniBand, Fibre Channel, and/or Ethernet) and the first switch network circuitry 102-1, 102-2. The I/O circuitry provides protocol conversion, including packet format conversion, during high-speed data communication between the external network 104 and the first switch network circuitry 102-1, 102-2. In some embodiments, the external network I/O interface circuits 112-1 to 112-4 are implemented as network interface cards commonly referred to as NICs, which include circuits that are configured to transform packets to suitable formats as they pass between the external network 104 and the routing networks 102-1, 102-2.

The storage I/O interface circuits 114-1 to 114-4 manage the distribution of data across the Flash storage circuits 110-1 to 110-8. In some embodiments, the storage I/O interface circuits are configured to implement a file system used to control how data is stored in and retrieved from storage devices. In some embodiments, the storage I/O interface circuits 114-1 to 114-4 are implemented as RAID controllers configured to organize data across multiple storage devices, such as Flash storage devices 110-1 to 110-8. The term RAID refers to data storage schemes that combine multiple disk drive components into a logical unit for the purposes of data redundancy and performance improvement. Persons skilled in the art will appreciate that Flash storage, sometimes referred to as solid-state drive (SSD) is a data storage device using integrated circuit assemblies as memory to store data persistently. Each of the storage access switch networks 108-1 to 108-4 provides point-to-point connections 124-1 to 124-4, respectively, using a serial protocol that moves data to and from the Flash storage devices 110-1 to 110-8. In some embodiments, the storage access switch networks 108-1 to 108-4 use a protocol that includes the SAS (Serial Attached SCSI) protocol. In general, according to the SAS protocol, there are three types of SAS devices: initiators (e.g., RAID controllers), target storage devices (e.g., Flash circuits), and expanders. An initiator device attaches to one or more target storage devices, to create a SAS domain. In some embodiments, the storage I/O interface circuits, implemented as RAID controllers, act as SAS initiators. In accordance with some embodiments, the Flash storage circuits 110-1 to 110-8 act as SAS targets. Using expanders (e.g., low-cost, high-speed switches), the number of targets attached to an initiator can be increased to create a larger SAS domain.

Communication paths 126-5, 126-6 couple storage I/O interface circuit 114-1 to exchange data with storage access switch networks 108-1 and 108-3. Communication paths 126-1, 126-2 couple storage I/O interface circuit 114-2 to exchange data with storage access switch circuits 108-2 and 108-4. Communication paths 126-3, 126-4 couple storage I/O interface circuit 114-3 to exchange data with storage access network circuits 108-3 and 108-1. Communication paths 126-7, 126-8 couple storage I/O interface circuit 114-4 to exchange data with storage access switch networks 108-4 and 108-2. Thus, all Flash circuits 110-1 to 110-8 are accessible via the first internal network circuit 102-1 via the storage I/O interface circuits 114-1, 114-2 coupled to it, and all Flash circuits 110-1 to 110-8 are accessible via the second internal network circuit 102-2 via the storage I/O interface circuits 114-3, 114-4 coupled to it.

In some embodiments, the first and second packet processing circuits 118-1, 118-2 are implemented as field programmable gate array (FPGAs). FPGA circuitry often can impart services with less latency delay, and therefore, faster than a typical general purpose management processor, for example, since the programmable logic can be programmed in advance to dedicate specific hardware circuitry to provide the services. Programmable hardware logic such as FPGA circuitry often can perform operations faster than, for example, a general purpose processor which often uses software interrupts to transition between different operations. Alternatively, in accordance with some embodiments, one or more of the packet processing circuits can include a special purpose processor, an application specific integrated circuit (ASIC), or an array of processors configured to run software to perform a given service.

The first and second packet processing circuits 118-1, 118-2 of FIG. 1 also are directly coupled to each other so that the same data can be cached at both. In some embodiments, a communication path 120 coupling the first and second programmable logic circuits includes a circuit connection compliant with a high speed network communication protocol. In some embodiments, the communication path 120 complies with the Ethernet protocol.

The first programmable logic circuit 118-1 is operably coupled to first cache storage circuitry 106-1. The second programmable logic circuit 118-2 is operably coupled to second cache storage circuitry 106-2. In some embodiments, the first and second cache circuits include DRAM circuits. More particularly, in some embodiments the first and second cache circuits include Flash-backed DRAM circuits in which Flash circuits are coupled to stored data persistently in the event of failure of a corresponding DRAM circuit.

Cache Memory

Cached data and cache metadata can be stored in Flash backed-up DRAM included in cache storage circuitry 106-1 and 106-2. FIG. 3 is a block diagram illustrating a dual inline memory module (DIMM) containing DRAM and flash memory, which in accordance with some embodiments, can be plugged into a standard DDR3 DIMM socket. In some embodiments, during operation the module behaves similar to a standard DDR3 DRAM DIMM, however, upon the occurrence of the events specified below, data is copied between the DRAM and flash. The entire contents of on-board DRAM are written to flash upon the occurrence of any of the following:

-   -   If configured to do so: A drop in supply voltage is detected by         an on-board circuit. Super-capacitors may supply power to the         Cache module to complete this operation.     -   A SAVE command is received via the I2C interface on the DIMM         socket.     -   An interrupt is received via a pin on the DIMM socket.

The contents of flash are written to DRAM upon occurrence of any of the following events:

-   -   If configured to do so: After power up if a valid saved image is         detected in flash.     -   A RESTORE command is received via the I2C interface on the DIMM         socket.

Reconfigurable Circuitry

The packet processing circuits 118-1 and 118-2 of FIG. 1 can be configured to include a cache controller to provide cache management services. The circuitry of a cache controller may include a state machine to perform the functions described. This offloads cache control functions to hardware from software to increase cache speed. A cache controller may also be capable of accommodating other hardware accelerators for off-loading CPU software functions to hardware. Some examples of these functions include encryption/decryption, duplication/de-duplication, compression/de-compression, processing, replication, and snapshot.

The software driver (e.g., of a management processor) and cache controller allow for 1:1 redundancy of data storage between cache memory and system flash memory. Data redundancy may also be applied to the cached data by having two cache controllers operate together to mirror the cache data. If one cache controller fails the other cache controller can operate independently, and cached data is preserved without any data loss.

FIG. 4 shows an example of interfaces to a cache controller. A cache controller can include two PCIe interfaces, PCIe CH0 and PCIe CH1. A cache controller can appear as a PCIe end-point on two RCs (one end-point per RC). One cache controller and one PCIe switch may reside on one switch card or switch board of a switch fabric. Hosts on the server boards can communicate with the cache controllers via the PCIe switches. A PCIe interface may provide a raw bandwidth of 64 gigabits/sec (64 Gbps). Channel 0 (PCIe CH0) can be electrically coupled to a PCIe switch local to the switch board of the cache controller and Channel 1 (PCIe CH1) can be electrically coupled to a PCIe switch remote from the switch board of the cache controller. A cache controller can include a serial link for inter-cache controller communication between switch boards. In an example, a serial link can include four lanes and have aggregate bandwidth of 10 Gbps.

Each cache controller may include an interface to DRAM that includes the storage space of the cache modules. In the example shown in FIG. 4, the cache controller includes two interfaces labeled DDR3 CH0 and DDR3 CH 1. Two dual in-line memory modules (RDIMMs) can be connected to the cache controller for data and cache table storage. In certain variations, one RDIMM can store 8 gigabytes (8 GB). Other sizes are possible such as 16 GB or 32 GB for example.

FIG. 5 shows a flow diagram of an example of a method 500 of managing access to cache memory using two (e.g., redundant) cache controllers. Each cache controller performs cache management tasks for a cache memory. The cache memory may include one or more cache modules 106-1 and 106-2 shown in FIG. 1. At block 505 of FIG. 5, a first cache command is posted in a first command buffer by a first host device and a second cache command is posted in a second command buffer by a second host device. A host device may be a management processor 116-1 and 116-2 in FIG. 1 and a cache command may be posted by a host executing at the management processor. The command buffers may reside in the host devices and can be separate from the cache controller.

Referring again to FIG. 5 at block 510, the first cache command and the second cache command are loaded into a buffer memory of the first cache controller, and the first cache command and the second cache command are loaded into a buffer memory of the second cache controller. In some examples, the writing of the command buffers initiates the performance or action of a cache controller. At block 515, the order of execution of the first and second cache commands is synchronized in the first and second cache controllers. The synchronizing of the order of execution of the first and second cache commands can include communicating an indication between the first and second cache controllers that the loading of the first and second cache commands is completed. The synchronizing can include communicating the size of the loaded cache commands between the first and second cache controllers. This information may be used by the two cache controllers to coordinate the setting of a pointer for the command buffer memories.

At block 520, both the first and second cache commands are performed using both the first cache controller and the second cache controller. The contents of the data storage of the first cache memory and the second cache memory are substantially identical after the first and second cache commands are performed by both of the first and second cache controllers. This results in redundancy of the cache memory contents. If one cache memory fails or cache data is otherwise corrupted, a duplicate exists. The non-failing cache can operate independently and cached data is preserved without loss of data.

FIG. 6 shows a block diagram of portions of an example of a cache controller 650. Also shown are representations of a left host device 665 a and a right host device 665 b. An example of a host device is a management processor 116 in FIG. 1 that may be implemented as a server board. The designation left and right are arbitrary and the designation could also be, among other things, a number such as host device 0 and host device 1. The host devices also show representations of a cache command memory 667 a,b or command circular ring buffer, read data buffers 668 a,b and write data buffers 669 a,b.

In the example shown in FIG. 6, each cache controller 650 includes a cache memory interface 655 (labelled DDR3 I/F for “DDR3 DRAM DIMM Interface”). As shown in the example of FIG. 4, a cache controller 650 may have multiple interfaces to cache memories or cache modules. The cache controller 650 also includes an inter-cache controller communication link 683. The inter-cache controller communication link 683 may be a serial link that provides bidirectional communication with the other cache controller. The cache controller 650 includes two peripheral interfaces. A first peripheral interface 640 a is used to communicate with the first host device 665 a and a second peripheral interface 640 b is used to communicate with the second host device 665 b. In some variations, the peripheral interfaces 640 a,b are PCIe interfaces. A cache controller 650 and a routing network 102 of FIG. 1 can reside on a switch card, and hosts running on a management processor 116 communicate with the cache controller via the routing network. The first peripheral interface (PCIe-0) can be connected to a first host device via a PCIe interface local to the cache controller 650 and the second peripheral device (PCIe-1) can be connected to the second host device via a PCIe interface remote from the cache controller 650.

The cache controller 650 can include logic circuitry 605. In some variations, the logic circuitry includes hardware circuits that implement a state machine. The logic circuitry may be reconfigurable or non-reconfigurable. The logic circuitry controls operation of the cache controller 650. For instance, the logic circuitry may load a cache command from the cache command memory of the first host device and load a cache command from a cache command memory of the second cache controller. The logic circuitry may trigger the pulling and execution of a cache command when an indication is received that a cache command is loaded into at least one of the cache command memories 667 a,b.

Each host (e.g., left (L) host and right (R) host) posts cache commands to its own command ring buffer within memory of the host device. As a host posts cache commands to the command ring buffer, it updates a tail pointer of the ring to a register within the cache controller memory space. Each cache controller can include a first-in-first-out memory buffer 610 (FIFO) to store cache commands loaded or pulled from the first and second host devices. The cache controller 650 pulls cache commands from the command ring buffers into its on-board FIFOs. This can be done as memory-reads (MEMRD) to the host memory, with the length of the MEMRD command set to the PCIe maximum transmission unit (MTU) or the number of commands on the ring, preferably whichever is less. After each “command pull” the cache controller 650 waits for the other cache controller to perform the same command pull. The logic circuitry 605 can be configured to reorder cache commands received from the hosts and stored in the FIFO of the cache controller to match an execution order of commands of the FIFO of the other cache controller. This ordering ensures that the same sequence of cache command is performed by the cache controllers for commands from both of the left and right hosts.

The cache controller may include a first content addressable memory (CAM) 615 internal to the cache controller 650. The cache controller may include a second CAM 620 including associative memory configured to describe the contents of the cache memory. The first CAM is used to store one or more queues containing cache commands. These queues can be viewed as Virtual Queues (VQs) that include a sequence of cache commands for a specified cache entry index for the cache memory. After re-ordering in a FIFO, cache commands are placed within the VQs based on the command cache key. A cache key is used to locate an entry in the cache and is included in field of a command word to indicate the cache key used in the cache operation. If the command is a Cache Query command, the Cache Key field includes the number of entries returned by the Cache Query command and an index of the last returned valid cache entry. A cache key may be assigned its own queue, and the queues may be serviced in a round-robin fashion. After the cache keys are assigned to queues, the cache command keys of the received commands are matched to the keys assigned to the queues. If a match is found for the key of the received command, the key is placed in the matching queue. If no match is found for a key of a received command, a new queue is created if space allows. When all commands of a queue are completed, the queue is deleted. The first CAM 615 can be relatively small (e.g., large enough to hold 256 command entries or 64 queues). The second CAM 620 can be relatively large (e.g., 4 million entries) and can be used to store metadata for the cache memory contents.

After a cache command operation is fully completed and data is written to either DRAM or host memory, the cache controller may write a status word to the host memory indicating the status of the completed command (an error status can be reported at this stage to indicate a failure during command execution). Once each host receives the status word from both cache controllers, it can free the data buffer memory within its memory space. Although only one cache controller returns cache read data to the host for cache read command, both cache controllers should return the same completion status indication to the host. It is an error condition when only one cache controller returns completion status or if the returned completion statuses are not the same.

FIG. 7 shows a diagram of an example of a sequence of communications for loading cache commands in cache controllers. The Left Host (L-Host) posts cache commands to its command ring buffer and the Right Host (R-Host) posts commands to its command ring buffer. At 705, the Left Host updates a tail pointer (TL_PR update) of the command ring buffer to a register within the left cache controller memory space (L-FPGA-RO-FIFO) and Right Host updates a tail pointer of the command ring buffer to a register within the right cache controller memory space (R-FPGA-RO-FIFO). The tail pointers may be updated using a peripheral interface between the hosts and the cache controllers. The sequence shows the Left Host sending a tail pointer first. This may happen when the Left Host is given priority over the Right Host. At 710, the size of the loaded cache commands is communicated between the left and right cache controllers. This communication may be implemented using a serial link between cache controllers.

Each host posts cache commands to the companion cache controller. At 715, the Left Host updates a tail pointer (TL_PR update) of the command ring buffer to a register within the right cache controller memory space (R-FPGA-RO-FIFO) and Right Host updates a tail pointer of the command ring buffer to a register within the left cache controller memory space (L-FPGA-RO-FIFO). At 720, the size of the cache commands is communicated between the left and right cache controllers and matched by the cache controllers.

At 725, each cache controller sends a command pull request to each host and cache commands are pulled from each host device buffer memory by each cache controller. At 730, an indication the completion of loading of the first and second cache commands is communicated between the left and right cache controllers. At 735, the cache controller transfers the cache commands into virtual queues. The cache commands are shown queued by the left cache controller first and right cache controller second.

The cache commands include cache read (CR) and cache write (CW) commands. FIG. 6 shows read data buffers 668 a,b for the host devices 665 a,b respectively. For each cache read operation, 4 kilobytes (4 KB) of data is read from the DRAM and returned to the host memory. The block 4 KB is only one option for the block read size and other block sizes are possible. In some examples, the cache controller 650 includes direct memory access engine 625 (DMA). The block of read data is returned to the host memory from the DRAM by the DMA engine. A CR command includes one or more bits to indicate which cache controller will return the read data. If there are two cache controllers designated as left and right, this may include one L/R bit to designate left or right. This prevents both cache controllers from writing the same data to the same read buffer in host memory. Preventing the redundant write preserves the bandwidth at the PCIe and DRAM interfaces.

FIG. 8 shows a diagram of an example of a sequence of communications for cache read commands performed by the left cache controller. The example shown involves a cache hit for a CR command from the left host. At 805, the CR command is executed out of the virtual queue (e.g., in the first CAM 615 of FIG. 6) and CAM is searched (e.g., the second CAM 620) to determine the status of the data in the cache memory. At 810, a block read of cache memory is performed by the left cache controller. The block read can be performed using the DDR3 I/F to DRAM. A block of data (e.g., a 4 KB block) is transferred to buffer memory (e.g., read buffer memory 668 a in FIG. 6) and a status word is written to memory of the Left Host. At 815, a second block read is performed for the Left Host. The example in FIG. 8 also includes a CR command for the Right Host, and shows two block reads 820 performed by the left cache controller and transferred to memory of the Right Host together with status words for the read operations.

FIG. 9 shows a diagram of an example of a sequence of communications for cache read commands performed by the right cache controller. As explained above, both cache controllers load the cache commands from each of the Left and Right hosts, and both cache controllers perform the commands. Only one cache controller may return cache read data to a host for a cache read command, but both cache controllers return a completion status word.

Returning to FIG. 6, write data buffers 669 a,b are shown for host devices 665 a,b respectively. Cache write data can be pre-fetched from the write data buffer 669 a,b. In certain variations, the pre-fetching is performed by the DMA 625. Pre-fetching ensures no waiting at the DRAM interface for a PCIe MEMRD operation. The write data is written to the DRAM once the DRAM address is obtained from second CAM 620.

FIG. 10 shows a diagram of an example of a sequence of communications for cache write (CW) commands performed by the left cache controller. After cache commands are pulled from the hosts and transferred from the FIFOs to the virtual queues, a data pull request originates from a virtual queue. At 1005, two data pull requests are sent to the Left Host and CAM is updated accordingly. Each request is for a block of data (e.g., a request for 4 KB of data). At 1010, data pull requests are sent to the Right Host. At 1015, a block of data is pulled by the left cache controller from memory of the Left Host. The data is written to DRAM of cache memory and a completion status word is returned to the Left Host. At 1020, a second block of data is pulled by the left cache controller from memory of the Left Host and a second completion status word is returned to the Left Host when the data is written to cache memory. At 1025, two blocks of data are pulled by the left cache controller from the Right Host. The data is written to cache by the left cache controller and completion status is returned to the Right Host.

FIG. 11 shows a diagram of an example of a sequence of communications for CW commands performed by the right cache controller. Cache write commands from the Left Host and the Right Host are performed by both the left and write cache controllers. This results in duplicate cache data in the cache memories.

Cache Commands

FIG. 12 shows an example of a format for cache commands. The field labeled CMD can include 4 bits and designates the command as one of the following: Cache Read (CR), Cache Read with Modify (CRM), Cache Write (CW), Cache Modify (CM), Cache Evict (CE) and Cache Query (CQ). The field labeled L/R bit designates the cache controller that returns the data. For example, if the value of the bit is “0,” read data is returned to the host by the left cache controller, and if the value of the bit is “1,” read data is returned to the host by the right cache controller. The field can be expanded if there are more than two cache controllers that can return cache data.

A cache command can include a dirty bit (labeled DRTY in FIG. 12). A cache write command sets the state of the dirty bit in the cache metadata to the state of the DRTY bit in the command. The DRTY bit can be used to properly implement cache access synchronization by the multiple hosts and redundant cache controllers. This concept can be illustrated by the following scenario. Assume the Left Host has just completed a read from the flash memory as a result of a cache miss. Also assume the Right Host has received a write request corresponding to at least a portion of the same data as the Right Host from an application program, and the Right Host determines to write the data to cache memory. The Right Host writes the data to cache memory with DRTY bit active (e.g., set to one) in the cache command. The Left Host may try to retain a copy of the data from flash memory by writing the data to cache. It will attempt a cache write with the DRTY bit set inactive (e.g., set to 0). If allowed to go through, the cache write operation from the Left Host would cause the data from the Right Host to be lost. Consequently, the cache controller will prevent the cache write by the Left Host. The cache controller may return a completion status that indicates the entry was found but not written.

The cache command can include a Cache Write Mask filed (labeled CW-MSK in FIG. 12) that can comprise 8 bits and is used for disabling writes to blocks of 512 bytes when data is written to cache memory during execution of the Cache Write command.

The cache command can include a sequence field (labeled SEQ#) that can comprise 17 bits and is used as a pointer to the original command. Software can use the field as an index into the cache command ring buffer. The Eviction Candidate bit field (labeled EC) indicates a cache entry has been tagged for eviction. The Eviction Candidate bit can only be set to an active state or 1 by a cache read modify command (described below). If a cache write command is executed by a cache controller the Eviction Candidate bit is set to an inactive state or 0. The “Cache Key” or “Cache Query Index and Entry Count” field can comprise 96 bits. As used by all commands except cache query command, the Cache Key filed indicates the Cache Key used in the cache operation. For the Cache Query command, this field indicates the starting entry index and maximum number of valid cache entries to be returned by the cache controller to the host.

The Host Buffer Address field can comprise 64 bits and indicates the physical address of the Host Data buffer in the host memory. It can be used as the address of the data for the cache block transfers to and from the host memory. The Host Status Address can comprise 32 bits and is used as an offset for the status information once a cache command is completed. The Host Status Address can be added to the Host Buff Address to form a 64 bit physical address within the host memory. The Cache Key Aging Alias can comprise 32 bits and is used in an algorithm to determine cache eviction.

Steps taken by a cache controller are described below for cache commands including a cache read, a cache read modify, cache write, cache modify, cache evict, and cache query. For a Cache Read (CR) command, if the there is a hit on the data in cache (a match in a cache tag), the cache controller will perform a search of the corresponding cache metadata in CAM. The address of the data is returned by the CAM response and the data is then read from cache memory (DRAM). The data may be read as a block of data by a DMA bock transfer to the host memory. The cache controller returns the completion status of the operation to the host memory. If there is a cache miss, the cache controller returns the completion status with an indication of the miss to the host memory.

The Cache Read Modify (CRM) command is similar to the Cache Read command except that it will set the Eviction Candidate bit in the cache metadata. The host originating the command does not have to be the “owner” of the read cache entry to issue this command (i.e., either host can issue this command). This command is the only command which can set the EC bit to an active state (e.g., set the EC bit to a 1).

For a cache write (CW) command, the cache controller performs a pre-fetch of a block of write data (e.g., 4 KB) from the host memory. The cache controller also looks for a hit on the key in second CAM 1520. The current state of the DRTY bit within the cache metadata is compared with the state of DRTY bit within the cache command. If the DRTY bit is set to 1 in the cache metadata and set to 0 in the cache command, the cache controller will return a “Failed” status. Otherwise, the DRTY bit in the cache metadata is set to the same state as the DRTY bit in the cache command and the pre-fetched block of data is written to the DRAM at the address obtained by CAM search command.

In the case of a cache miss, the cache controller inserts the cache key in the cache metadata and sets the DRTY bit and the ownership bit of the cache line in the cache metadata. The DRAM address of the data is obtained as part of the CAM response and the cache controller writes the pre-fetched block of data to the DRAM at the address given by CAM. The cache controller then sends the command completion status to the host memory to indicate buffer can be freed. The command completion status can be sent using the cache controller DMA.

For a Cache Modify (CM) command, the cache controller looks for a hit on the data in cache. If there is a cache hit, the cache controller performs a search of the cache metadata in CAM. If the Eviction Candidate bit is set then the DRTY bit of the metadata for the cache entry is cleared; otherwise the cache controller returns a “Failed” status. In response to the cache hit, the cache controller returns a cache completion status of “hit” to the host memory. In the event of a cache miss, the cache controller sends a cache completion status of “miss” to the host memory. The command completion status can be sent using the cache controller DMA.

For a Cache Evict (CE) command, the cache controller looks for a hit on the data in cache. If there is a cache hit, the cache controller performs a search of the cache metadata in CAM. If the host issuing the CE command is the owner of the cache entry and the Eviction Candidate bit is set for the entry, the cache controller deletes the cache entry and the cache key. This is done regardless of the state of the DRTY bit in the metadata. A programming option can be provided to disable the checking of the ownership bit before deleting the cache entry. In response to the cache hit, the cache controller returns a cache completion status of “hit and success” or “fail” to the host memory. In the event of a cache miss, the cache controller sends a cache completion status of “miss” to the host memory.

The Cache Query (CQ) command instructs the CAM to return N valid cache entries, where N is a positive integer. The cache entries start from the specified cache entry index in the command. If there are no more valid entries in the cache before N is reached, CAM will stop returning entries once the last entry is reached. The Cache Key field in the CQ command is used to specify the number of valid cache entries to be returned (N), as well as the starting index of the last valid entry. The Host Buffer Address field of the command is used to specify where in host memory to place the returned cache entries. The L/R bit of the command indicates whether the Left of Right cache controller is to scan the cache and return the valid entries. The cache controller returns a completion status that indicates how many cache entries were returned and whether all of the entries have been scanned. The completion status also indicates the index of the last valid entry returned.

The cache commands described have referred to returning a completion status word. FIG. 13 shows an example of a format for a completion status word. The Completion Status Field (labeled Cmpl Status in FIG. 13) indicates the completion status of the cache command. The completion status field may include one or more the following: a Miss bit to indicate that the cache command resulted in a cache miss, an Error bit to indicate that execution of the command resulted in an error, and a Fail bit the cache command was not successful. A command failure has different meaning depending on the cache command. For instance, a Cache Evict command may fail due to the EC bit being 0. The completion status field may include a separate Insert-Fail bit to indicate the failure of a Cache Write command to insert a cache entry due to the cache being full in the case of a cache miss. The completion status field may also include one or more cyclic redundancy code (CRC) bits to indicate that the data read as result of a CR or CRM command had a CRC error. The cache controller may include CRC generating circuitry, and the CRC error may indicate that the CRC generated while reading the DRAM did not match the CRC stored in the cache metadata. The Completion Status Field may include a Q-End bit to indicate when the cache entries returned for a Cache Query command are the last valid entries in the cache. This bit is only applicable to the Cache Query command

The completion status word may include additional fields. In some examples, the completion status word includes a “Furthest Cmpl Ptr” field that contains a pointer to the furthest completed cache command. The completion status word may include a CMD field that contains the original cache command for which the returned completion status corresponds, and may include a DRTY field of one bit to indicate the content of the “dirty” bit within the cache metadata. The DRTY field is only valid if the cache operation resulted in a hit. This bit is undefined for a Cache Query command.

The completion status word may include an OWNER field. The OWNER field may comprise one bit to indicate the “owner” of the cache entry (e.g., a cache line). A 0 in the field may indicate the left host as the owner and a 1 in the field may indicate the right host. The owner is the host whose Cache Write command resulted in an insert of the cache entry (e.g., as a result of a Cache Write miss). The OWNER field is used to for evicting a cache entry and for clearing the DRTY field in the metadata. For instance, a cache entry can only be evicted using the Cache Eviction command when the host issuing the Cache Eviction command is the owner of the cache entry. A DRTY bit in the metadata for a cache entry can only be set by a Cache Write command and the cache owner is determined at the time of the cache write. The DRTY bit can only be cleared by a Cache Modify Command sent from the owner of the cache entry when the Eviction Candidate bit is set. The OWNER field is undefined for the Cache Query command.

The completion status word may also include one or more of a L/R field, and an EC field. The L/R field can comprise one bit to indicate whether the Left or Right cache controller is returning data for the Cache Read, Cache Read Modify, and Cache Query commands. For instance, the Left cache controller will return data to the requesting host if the field is set to 0, and the Right cache controller will return data to the requesting host if the field is set to 1. The EC field may include a bit that indicates the value of the Eviction Candidate bit within the cache metadata when the cache command was executed. This bit is undefined for the CQ command.

The completion status word may include a sequence number (SEQ#) field. This field contains the original sequence number in the corresponding cache command. The contents of the SEQ field can originally be set by software as a cache command index in the command ring. This field can be used by software to quickly find the corresponding entry in the command ring buffer.

The completion status word may also include a Cache Key field and a Key Aging field. Like the command word, the Cache Key field contains the cache key within the cache metadata. If the cache command is a Cache Query command, this field indicates the number of entries returned in response to the Cache Query command and may also indicate the index of the last returned valid entry. The Key Aging field is a value stored in the cache metadata to track cache entry aging.

Cache Entry Aging

Cache entry aging determines what data to displace from a cache module. Data to be displaced or evicted from a cache module can be determined according to a set of cache eviction rules or a cache eviction algorithm. The cache aging algorithm can be performed using driver software residing in a host device 665 a,b of FIG. 6. Information required by the driver software can be stored in hardware within the cache metadata.

According to some examples, the cache aging algorithm implements an empty list and a least recently used (LRU) linked list using memory of the host device. The empty list includes pointers to free memory space where the data structures of the LRU list can be stored. The number of entries in the empty list may depend on the amount of cache memory allocated to the host device. If there is more than one host device (e.g., two host devices 665 a,b as in the example shown in FIG. 6) each host device is allocated a portion of the cache memory. The LRU list includes cache keys for the entries stored in the cache memory. The LRU list orders the cache entries according to the amount of activity or access involving the cache entries. Activity can be tracked according to a number of hits or the hit rate of an entry or according to latency between accesses to an entry.

FIG. 14 is a flow diagram of an example of a method 1400 of implementing cache entry aging. At 1405, a cache command is posted in a command buffer (e.g., cache command memory 667 a or 667 b in FIG. 6) by a host application of a host device. As shown in FIG. 12, a cache command includes a cache key and a key aging alias. At 1410, logic circuitry of the cache controller loads the cache command into buffer memory of the cache controller when a cache command is posted in the command buffer. At 1415, the cache controller determines whether there is a match for the cache key in a command queue of the cache controller (e.g., second CAM 620 in FIG. 6). The cache controller then performs the cache command accordingly.

At 1420, cache completion status information is returned from the cache controller to the host application. If a match for the cache key was found in the command queue (a cache hit), the cache completion status includes the value of the key aging alias in cache metadata. If a match for the cache key was not found in the command queue (a cache miss), the cache completion status includes the value of the key aging alias provided by the host application.

At 1425, the cache key is at the end of a least recently used (LRU) linked list when the cache completion status information includes the value of the key aging alias provided by the host application. The cache key is moved to the end of the LRU when the cache completion status information includes the value of the key aging alias in cache metadata.

FIG. 15 illustrates the interaction of the empty list 1571 and the LRU linked list 1573 with a command ring buffer 1567 of the host device and cache metadata within the cache memory of the cache controller for the case of a cache miss during a cache write command. When the host application posts a cache command into the command ring buffer 1567, an entry from the empty list is included 1572 in the key aging alias field of the command as shown in the example of FIG. 12. The cache controller then looks for a match for the cache key of the command. Because of the cache miss for the write data, a match for the cache key is not found and the key aging alias provided by the host application is inserted 1574 in the cache metadata. The cache completion status information includes the value of the key aging alias provided by the host application. The cache key is stored by the host application at the location pointed to by the key aging alias. The cache miss results in a new entry 1576 at the end or tail of the LRU list 1573 that includes the cache key.

FIG. 16 illustrates the case of a cache hit for a cache write or a cache read. As in the example of FIG. 15, an entry from the empty list is included 1672 in the key aging alias field of the command when the host application posts a cache command into the command ring buffer 1667. When the cache controller finds a match for the cache key of the cache command in the cache metadata, the cache controller discards 1674 the key aging alias in the cache command. The cache completion status information includes 1676 the value of the key aging alias in the cache metadata within the cache memory. The host application returns 1678 the key aging alias of the command ring buffer to the empty list 1671 and the cache key is at the location pointed to by the key aging alias. The cache hit results in an entry of the LRU containing the cache key being moved to the tail of the LRU list.

As explained above, the system 100 can include two host devices and two cache controllers. Host Applications of both host devices can post cache commands. Both of the cache controllers pull commands from the command buffers of both the host devices. For instance, a first cache command can be posted by a first host application (e.g., of the first host device) and a second cache command can be posted by a second host application (e.g., of a second host device). The logic circuitry of the second cache controller loads or pulls the first cache command into its buffer memory, performs the first cache command, and returns cache completion status information to the first host application for the first cache command.

Cache commands are performed using both the first cache controller and the second cache controller. This provides redundancy in cache management of the system. Referring to FIG. 6, the cache controllers include an inter-cache controller communication link 683 that provides bidirectional communication. The logic circuitry of a cache controller reorders cache commands in the buffer memory according to the cache key and to match the order of cache commands in the buffer memory of the other cache controller.

As explained previously, once a host application receives the cache completion status information from both of the first and second cache controllers, it can free the data buffer memory within its memory space. If the first cache command is a cache read (CR) command each of the first cache controller and the second cache controller perform the cache read and return cache completion status information to the first host application, but only the first cache controller returns cache read data to the first host application. Both of the host applications implement and maintain a copy of the empty list and LRU linked list in memory of its host device.

According to some examples, the driver software of a host device implements an eviction list as part of the cache aging algorithm. The eviction list includes the candidate cache entries for deletion from the cache memory. A specified number of entries at the head of the LRU list are moved to the eviction list when the number of entries in the empty list is less than or equal to a specified threshold number of entries. The candidate cache entries are sent to the cache controller with the cache evict (CE) command described previously, and the cache controller evicts the cache entry and the cache key from the cache memory.

The host application of a host device determines when a cache entry will be evicted. The host application is only allowed to evict those entries it owns (e.g., as indicated by an ownership bit). For instance, a first host application may post a first cache command and a second host application of a different host device may post a second cache command that results in a cache entry being written into cache memory. The logic circuitry of the first cache controller loads the second cache command into its buffer memory and updates the cache metadata to indicate ownership of the cache entry by the second host application. The cache entry is evicted by the logic circuitry in response to a request to do so by the second host application when the cache metadata still indicates ownership of the cache entry by the second host application.

As part of the eviction of a cache entry, the host application generates a cache command (e.g., a CRM command) that includes a request to read a cache entry from cache memory according to the LRU linked list of the host application (e.g., the entry is at or near the head of the LRU linked list). The cache command indicates that the entry is a candidate for eviction (e.g., by setting an Eviction Candidate bit in the command). When the read data is returned to the host application, the host application writes the cache entry to flash memory 110 that is separate from the cache memory 106.

The cache metadata includes an indication that the read cache entry has been modified from what is stored in flash memory (e.g., a dirty bit). When the host application writes the cache entry to flash memory as part of an eviction of the cache entry, the host application may generate a request for the cache controllers to clear the indication or the dirty bit. The cache controllers return cache completion status information to the requesting host application when the command is performed. The cache controllers may return a failed completion status to the requesting host application when the requesting host application is not the owner of the cache entry. The cache controllers may also return failed completion status to the requesting host application when the requesting host device is attempting to evict a cache entry but the indication of the cache entry as an eviction candidate has been cleared (e.g., an EC bit is cleared) because the other host application has modified the cache entry.

Cache entry aging and eviction has been described using a least recently used algorithm as an example. Other algorithms can be used to implement cache aging, such as by tracking most recently used entries or by using an adaptive algorithm that changes the specified threshold number of entries used to initiate cache eviction.

Overview of Cache Management Service

FIGS. 17A-17C are illustrative drawings showing example flow diagrams representing caching principles, policies, or rules implemented by the software driver (e.g., of a host on a server board) and cache controller according to some embodiments. The software driver performs cache capacity management operations to ensure that the cache modules always have sufficient capacity to handle I/O requests. If cache capacity is not adequately monitored and optimized, for example, there may not be enough space to fully handle a new write request. Cache management for cache modules are performed independently of each other.

Referring to FIG. 17A, it is determined whether it is time to perform cache capacity maintenance at block 802. Maintenance may be a continuous background operation, a periodic background operation, or on a need basis type of operation. Maintenance frequency can be a system setting, user setting, or dynamic setting based on current operating conditions of the system 100 of FIG. 1. If maintenance is initiated (yes' branch of block 802), then at block 804 the cache controller determines whether the current data storage capacity of the cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller of the packet processing circuit 118-1 and 118-2) is at or above a pre-set maximum capacity level. The pre-set maximum capacity level is a certain value that is pre-set by the system 100 or user, and represents the portion of the total data storage capacity of the cache modules that can be occupied while having a “safe” amount of available space in case, for example, an unexpectedly large write request is received. Examples of pre-set maximum capacity level include, but are not limited to, 70%, 80%, or some other value. In some embodiments, the pre-set maximum capacity level may be adjusted over time as more system usage information becomes available. Instead of expressing the pre-set maximum capacity level as a percentage of the total data storage capacity, for example, it is understood that it can be expressed as a minimum available or reserved free space.

If the current cache capacity is below the pre-set maximum capacity level (‘no’ branch of block 804), then the flow diagram returns to block 802. Otherwise the current cache capacity is too close to the pre-set maximum capacity level, and some of the stored data needs to be moved to the flash modules 110 and evicted or erased from the cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller) (yes' branch of block 804).

Next at block 806, the cache controller determines what data to displace from the associated cache modules according to a set of cache eviction rules, such as according to a Least Recently Used algorithm for example that evicts data that is the least used. Data that is “pinned” stays within the cache module based on a user specified directive.

Once the cache line(s) to empty are identified, at block 808 the cache controller sends data stored in those cache line(s) to the flash modules 110-1 to 110-8 for storage. Such data is erased, emptied, or evicted from those particular cache line(s) at block 810. The flow diagram then returns to block 802. Thus, the cache capacity of cache modules 106-1 or 106-2 (depending on which set of cache modules is associated with the given cache controller) is maintained at or below the pre-set maximum capacity level. It is understood that blocks 808 and 810 may occur simultaneously of each other.

FIG. 17B illustrates operations performed by the software driver and cache controller of the cache management subsystem in connection with a data read request from any of the I/O circuits 112-1 to 112-4. At block 820, a software driver of a host (e.g., a general purpose management processor 116) receives a read request originating from one of the I/O circuits 112-1 to 112-4. Assuming normal operation, the software driver handles read requests originating from I/O circuit 112-1 to 112-4 using the cache controllers included in the packet processing circuits 118-1, 118-2. The data request includes a particular memory address location of the flash modules 110-1 to 110-8 at which the requested data is stored. A command for the data request may be posted in a command buffer of a host device for loading by the cache controllers.

Next at block 822, a look-up of the key in a cache table is performed to determine whether the requested data exists in the cache modules 106-1, 106-2. If a matching cache key is found (a cache hit, or the ‘yes’ branch of block 822), the cache controllers access the data corresponding to the matching cache tag from the cache module 106-1, 106-2 and sends the retrieved data to the software driver and the data is sent to the originating I/O circuit at block 824. The retrieved data is the requested data in the read request. The tracking metrics (e.g., according to a least recently used algorithm) is updated at block 826. If the retrieved data was previously written to the cache module 106-1, 106-2 (in a previous write request) and such data was not evicted from the cache module due to cache management operations (see FIG. 17A), then such data is present in the cache module for later access such as the present read request. When data is present in the cache module there is no need to retrieve the data from the flash modules 110-1 to 110-8. Data retrieval from a DRAM cache is significantly faster than from flash-based memory, upwards of a thousand times faster using cache than flash.

If no matching cache key is found (a cache miss, or the ‘no’ branch of block 822), the requested data is not present in the cache modules 106-1, 106-2 and is retrieved from the flash modules. At block 828, the cache controllers initiate retrieval of the requested data from the appropriate flash modules. Both cache controllers load the cache read command posted by the software driver of host. Both cache controllers perform the command, but only one cache controller may return the cache read data to the host.

Next at block 830, a system setting (or user specified setting) is checked to see whether the requested data retrieved from one or more of the flash modules 110-1 to 110-8, should be copied to the cache modules. If the system is set to not copy the data to cache modules (no′ branch of block 830) then the flow diagram proceeds to block 834. Otherwise the retrieved data is copied to the cache modules (yes' branch of block 830 and block 832). The retrieved data is also sent to the I/O circuit, one of 112-1 to 112-4 that made the read request at block 834. The associated tracking metrics (e.g., for a least recently used algorithm) are also updated at block 836.

FIG. 17C is an illustrative flow diagram representing operations performed by the software driver and cache controller of the cache management subsystem in connection with a data write request from any of the I/O circuits 112-1 to 112-4 in accordance with some embodiments.

At block 840, the software driver of a host receives a write request originating from one of the I/O circuits 112-1 to 112-4, and the request is then passed onto the cache controllers included in the packet processing circuits 118-1, 118-2 (assuming normal operational state of the system 100). The data request includes the data to be written as well as a particular memory address location of the flash modules at which the data is to be written.

At block 842, the software driver determines whether the data associated with the write request is exceptional. While the default rule is to store all data associated with write requests to the cache modules packet processing circuits 118-1, 118-2 and then at some later point in time copy data from the cache modules to the flash modules 110-1 to 110-8, one or more exceptions to the default rule may be implemented. One or more exception criteria may be a system setting or user specified setting. For example, the exception may comprise there being no exception to the default rule. As another example, data exceeding a certain size (e.g., data that if written to the cache modules may exceed the cache capacity or likely to exceed the pre-set maximum capacity level) may warrant storing directly in the flash modules without first storing in the cache modules. As still another example, the write request or the data associated with the write request itself may specify that the data will be rarely accessed (e.g., is archival data) or has a certain characteristic that warrants being stored directly in the flash modules 110-1 to 110-8 without first being stored in the cache modules 106-1, 106-2.

If the data associated with the write request is determined to be exceptional (yes' branch of block 842), then such data is sent to the flash modules for writing 110-1 to 110-8 at block 844. Otherwise the data associated with the write request is not exceptional (no′ branch of block 842) and operations are performed to write to the cache modules 106-1, 106-2. At block 846, the cache table is checked for a key of the cache data containing the same flash memory address location as provided in the write request.

If a matching cache key is found (yes' branch of block 846), this means that an older version of the data associated with the write request (or some data in general) is currently stored in the cache line(s) now intended for the data associated with the write request. The cache controllers facilitate overwriting the existing data at these cache line(s) with the data associated with the write request at block 848. Then the flow diagram proceeds to block 852. If no matching cache key is found (‘no’ branch of block 846), then the cache controllers facilitate writing the data associated with the write request to empty/available cache line(s) in the cache modules 106-1, 106-2 at block 850.

Next at block 852, the data associated with the write request is additionally copied to empty/available cache line(s) in the cache modules 106 associated with the other FPGA packet processing circuit. A cache command associated with the write request is posted by the software driver of the host and loaded by both cache controllers. In this way, the write request from the I/O circuit is formed into two identical requests, one going to the packet processing circuit 118-1 and the other to the packet processing circuit 118-2. Then the cache controller in each of the packet processing circuits can store the data associated with the write request (also referred to as write data) in its respective cache modules. At block 854, the associated tracking metrics are updated to reflect the addition of the data associated with the write request into certain cache line(s) of the cache modules.

Because flash modules 110-1 to 110-8 comprise the primary or permanent data storage medium for the storage system 100, the data associated with the write request, although already written to the cache modules 106-1, 106-2 (see blocks 848 and 850), is eventually written to the flash modules 110-1 to 110-8. Nevertheless, cache management of the system 100 is configured to intelligently perform data writes to the flash modules taking into account the characteristics of the flash modules. In order to prolong the usability of flash modules, which are limited to a certain number of writes before degrading, the cache management accumulates certain type of data corresponding to a plurality of write requests and then performs a single write of the accumulated data to flash modules rather than performing a write to flash modules for each write request. This means that if, for example, there are 25 write requests, instead of writing to flash modules 25 times or once for each of the 25 write requests, the data corresponding to these 25 write requests may be written at the same time and once (e.g., a single write operation) to the flash modules.

Accordingly, the cache management acts as a middleman between the I/O circuits 112-1 to 112-4 and flash modules 110-1 to 110-8 for every read and write requests from the I/O circuits. For all read and write requests, the presence of data associated with the read or write request in the cache modules 106-1, 106-2 is checked before the flash modules are involved. Based on the presence or absence of such data in the cache modules, the cache management performs optimization operations to complete the data requests significantly faster than is possible with flash modules alone. The cache management also prolongs the useful lifespan of flash modules by minimizing the number of writes to flash modules without sacrificing completeness of data being stored in the flash modules. Data associated with write requests are written to cache memory (prior to be written to flash modules) unless the data fits an exception. Data associated with read requests that are retrieved from the flash modules may or may not be written to cache modules corresponding to both packet processing circuits (depending upon a system or user setting). The cache management actively maintains the used storage capacity level of the cache modules at or below a pre-set capacity level (e.g., 70%, 80%, etc.) by evicting data stored in the cache modules that fit one or more eviction rules, as needed. An example of an eviction rule comprises evicting data that has the least amount of access (read or write) and moving it to the flash modules.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. One skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. Moreover, it will be appreciated that various modifications and alterations may be made by those skilled in the art without departing from the scope of the invention.

The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. 

What is claimed is:
 1. A system comprising: a first host device including a memory and a first host application, wherein the memory includes a command buffer and wherein the first host application is configured to post a first cache command in the first command buffer, wherein a cache command includes a cache key and a key aging alias; and a first cache controller including: a first peripheral interface configured for communication with the first host device; a buffer memory; content addressable memory (CAM) configured to store a command queue and cache metadata; and logic circuitry configured to: load the cache command from the command buffer of the first host device into the buffer memory; identify a match, if any, for the cache key in the command queue; perform the cache command; and return cache completion status information to the first host application, wherein the cache completion status information includes a value of the key aging alias in cache metadata when a match for the cache key is found and includes a value of the key aging alias provided by the first host application when a match for the cache key is not found, wherein the host application is configured to insert the cache key at the end of a least recently used (LRU) linked list stored in memory of the host device when the cache completion status information includes the value of the key aging alias provided by the first host application, and move the cache key to the end of the LRU when the cache completion status information includes the value of the key aging alias stored in cache metadata.
 2. The system of claim 1, wherein the memory of the host device is configured to store an empty list, wherein the empty list includes one or more pointers to memory in the host device where a data structure for the LRU list can be stored, wherein the first host application is configured to include an entry from the empty list in the key aging alias of the cache command.
 3. The system of claim 2, wherein the first host application is configured to indicate one or more cache entries for deletion from the cache memory when a number of entries in the empty list is less than or equal to a specified threshold number of entries.
 4. The system of claim 1, including a second host device having a memory and a second host application, wherein the second host application is configured to post a second cache command in a second command buffer, wherein the logic circuitry of the first cache controller is configured to: load the second cache command into the buffer memory of the first cache controller; update cache metadata to indicate ownership by one of the first host application or the second host application of a cache entry written into cache memory by the first or second cache commands; and delete the cache entry in response to a request by a requesting host application when cache metadata indicates ownership of the cache entry by the requesting host application.
 5. The system of claim 1, including a second host device having a memory and a second host application, wherein the memory is configured to store a second LRU linked list, wherein each of the first and second host applications is configured to: generate a request to a read of a cache entry from cache memory according to its LRU linked list; indicate the cache entry as a candidate for deletion from cache memory; and write the cache entry to flash memory separate from cache memory.
 6. The system of claim 5, wherein each of the first and second host applications is configured to generate a request to clear a cache metadata indication that the read cache entry has been modified from what is stored in flash memory; and wherein the first cache controller is configured to return a failed completion status to the requesting host application when at least one of the indication is not set or the requesting host application is not the owner of the cache entry.
 7. The system of claim 1, including a second cache controller including a second peripheral interface, a second buffer memory, and second logic circuitry configured to load the first cache command into the second buffer memory, perform the first cache command, and return cache completion status information to the first host application for the first cache command.
 8. The system of claim 7, wherein the cache command is a cache read command, wherein each of the first cache controller and the second cache controller is configured to return cache completion status information to the first host application, and wherein only the first cache controller is configured to return cache read data to the first host application.
 9. The system of claim 1, wherein the logic circuitry is configured to add the key aging alias to cache metadata when a match for the cache key is not found for a cache write command.
 10. The system of claim 1, wherein the first host device includes a server.
 11. A method of managing access to cache memory, the method comprising: posting a first cache command in a first command buffer by a first host application of a host device, wherein a cache command includes a cache key and a key aging alias; loading the first cache command into a buffer memory of a first cache controller; determining by the first cache controller whether there is a match for the cache key in a command queue of the first cache controller; returning cache completion status information from the first cache controller to the first host application, wherein the cache completion status information includes a value of the key aging alias in cache metadata when a match for the cache key is found and includes a value of the key aging alias provided by the first host application when a match for the cache key is not found; and inserting the cache key at the end of a least recently used (LRU) linked list when the cache completion status information includes the value of the key aging alias provided by the first host application, and moving the cache key to the end of the LRU when the cache completion status information includes the value of the key aging alias in cache metadata.
 12. The method of claim 11, wherein posting the first cache command includes posting a cache command having a key aging alias that includes an entry from an empty list, wherein the empty list includes one or more pointers to memory in the host device where a data structure for the LRU list can be stored, wherein the entry from the empty list includes a pointer to an entry in the LRU linked list which is stored in memory of the host application.
 13. The method of claim 12, wherein the first host application is configured to indicate one or more cache entries for deletion from the cache memory when a number of entries in the empty list is less than or equal to a specified threshold number of entries.
 14. The method of claim 11, including adding, by the first cache controller, the key aging alias to the cache metadata for a cache write command when a match for the cache key is not found in the command queue.
 15. The method of claim 11, including: posting a second cache command in a second command buffer of a second host device and loading the second cache command by a second host application into the buffer memory of the first cache controller; indicating, in cache metadata, ownership by one of the first host application or the second host application of a cache entry written into cache memory; and deleting the cache entry from cache memory in response to a request by a requesting host application when cache metadata indicates ownership of the cache entry by the requesting host application.
 16. The method of claim 15, including: requesting, by the first or second host application, a read of a cache entry from cache memory according to the LRU linked list; indicating the cache entry as a candidate for deletion from cache memory; and writing the cache entry to flash memory separate from cache memory.
 17. The method of claim 15, including: requesting, by a host application, to clear an indication in cache metadata that the cache entry has been modified from what is stored in flash memory; and returning a failed completion status to the host application when at least one of the indication was not set or the requesting host application is not the owner of the cache entry.
 18. The method of claim 11, including: loading the same first cache command into a buffer memory of a second cache controller; and returning cache completion status information for the first cache command from the second cache controller to the first host application.
 19. The method of claim 18, wherein loading the first cache command includes loading a first cache read command into a buffer memory of a first cache controller and a second cache controller, wherein the method includes performing the cache read operation by both the first cache controller and the second cache controller; returning cache completion status information from both the first cache controller and second cache controller; and returning cache read data from only the first cache controller.
 20. An integrated circuit (IC) including a cache controller, wherein the cache controller includes: a first peripheral interface configured for communication with a first host device; a buffer memory; content addressable memory (CAM) configured to store a command queue and cache metadata; and logic circuitry configured to: pull a first cache command from a command buffer of the first host device and load the first cache command into the buffer memory; identify a match, if any, for the cache key in the command queue; perform the cache command; and return cache completion status information to a first host application of the first host device, wherein the cache completion status information includes a value of the key aging alias in cache metadata when a match for the cache key is found and includes a value of the key aging alias provided by the first host application when a match for the cache key is not found.
 21. The IC of claim 20, wherein the cache controller includes a second peripheral interface configured for communication with a second host device, wherein the logic circuitry is configured to: pull the second cache command into the buffer memory; indicate, in cache metadata, ownership of a cache entry by one of a first host application of the first host device or by a second host application of the second host device; and delete the cache entry from cache memory in response to a request by a requesting host application when cache metadata indicates ownership of the cache entry by the requesting host application.
 22. The IC of claim 20, including an inter-cache controller communication link configured for bidirectional communication with a second cache controller, wherein the logic circuitry is configured to reorder cache commands in the buffer memory according to the cache key and to match an order of cache commands in a buffer memory of the second cache controller.
 23. The IC of claim 20, wherein the logic circuitry is reconfigurable. 